1. Field of the Invention
The present invention relates to a sheet feeder and an image forming apparatus and, more particularly, to a configuration in which a sheet feeding force can be prevented from being markedly reduced even in the case where an amount of stacked sheets is increased or sheets having a great density are stacked.
2. Description of the Related Art
An image forming apparatus, such as a printer or a copying machine, has been conventionally provided with a sheet feeder including a sheet feeding cassette serving as a sheet container of a feeding type, on which sheets are stacked, and a feeding portion which feeds the sheets contained in the sheet feeding cassette one by one in separation.
The above-described sheet feeder is exemplified by a sheet feeder provided with a feed roller which feeds sheets and a separation roller which separates the sheets in abutment against the feed roller. In a sheet feeding cassette disclosed in U.S. Patent Application Publication No. 2006/237895 A1, a lifter plate having sheets stacked thereon is disposed in a removable manner in a vertical direction, and further, the lifter plate is urged by a spring to bring the sheet into press-contact with the feed roller, thereby generating a feeding force. When the sheet is fed, the feed roller is rotated in press-contact with an uppermost sheet stacked on the lifter plate so that the uppermost sheet is fed. After that, the fed uppermost sheet is allowed to pass a nip interposed between the feed roller and the separation roller. In this manner, the sheets are fed one by one in separation.
In the above-described sheet feeder provided with the sheet feeding cassette such configured that the lifter plate is urged so as to generate the sheet feeding force, the weight of the sheets stacked on the lifter plate is increased in the case where an amount of the stacked sheets is increased or sheets having a great density are stacked. When the weight of the sheets is increased in such a manner, the weight reduces the resiliency of the spring, and therefore, a pressing pressure between the feed roller and the sheet is decreased, thereby raising a fear of instability of the sheet feeding force of the feed roller.
In view of the above-described present circumstances, the present invention has been accomplished to solve the above-described problems experienced in the prior art. Therefore, the present invention provides a sheet feeder and an image forming apparatus, in which a sheet feeding force can be prevented from being markedly reduced even in the case where an amount of stacked sheets is increased or sheets having a great density are stacked.